Method and system for the production of porous iron

ABSTRACT

Described are a method and a system for producing porous iron by means of a direct reduction process. In said method and system, a reducing gas is introduced into a DRI reduction device, and the direct reduction process is carried out therein. Coke oven gas and/or natural gas is/are reformed by adding gas, which contains steam and carbon dioxide and which is top gas from a DRI reduction device, and oxygen in a COG reformer so as to obtain reducing gas.

The invention presented here, relates to a method for the production of porous iron via a Direct Reduction Method, in which the reducing gas is introduced into a DRI reduction device and in which the Direct Reduction Method is carried out.

During the production of the porous iron in a DRI (Direct Reduced Iron) system based on natural gas as an energy source, the natural gas turns into hydrogen by supplying top gas that contains water vapor and carbon dioxide into catalytic tubes and by using the heat of a fuel gas in conjunction with a nickel catalyst and the carbon monoxide-containing reducing gas. Since natural gas prices have increased during the recent years and it is impossible to use unprocessed coke oven gas due to sulfur and higher hydrocarbons in the production of reducing gas in the existing DRI systems with catalytic tubes, new methods, such as the production of hydrogen or methane from coking gas, have been developed, in order to replace natural gas with coke oven gas or other cheaper methane-containing gases. The main drawback of this method however, in addition to an enormous expenditure of energy, is the enormous expenditure for the cleaning and the preparation of the coke oven gas, as well as for the disposal of the cleaning products. Furthermore, the environmental impact of the various toxic components plays a major role.

The invention presented here, has as its aim to provide a method such as one mentioned above, that can be implemented in a particularly efficient manner.

This aim is achieved by a method of the type indicated, in which the coke oven gas and/or the natural gas is/are reformed to the reducing gas by adding water vapor and carbon dioxide containing gas, which is top gas from a DRI reduction device, and of oxygen in a COG reformer.

A COG reformer refers to a coke oven gas reformer, i.e. a device for the conversion of coke oven gas. Based on this invention, a new simple method is provided, with which the cokinggas and/or the natural gas is/are treated without the use of great technical effort and is converted to reducing gas, that can be used to produce porous iron in a DRI reduction device. During the conversion of coke oven gas or natural gas to reducing gas, methane and higher hydrocarbons including the toxic components of these, are decomposed to form H₂ and CO.

By the partial oxidation of the higher hydrocarbons and methane by oxygen to H₂ and CO and by the combustion of a portion of the gas to water vapor and carbon dioxide, the temperature in the dome of the COG-Reformer is maintained at about 1000° C. At this temperature, a portion of the methane is decomposed from the reactions with the inserted water vapor and the steam that is formed in the COG reformer and the carbon dioxide into H₂ and CO. Even though, the higher content of water vapor and carbon dioxide in the reducing gas leads to a faster decomposition of methane and reduction of soot problems by the thermal decomposition of methane, whereby the COG reformer may be designed smaller, its content in the reducing gas should not be too high, to maintain the required reduction potential of the reducing gas.

In further development of the invention, the reducing gas is partly produced in the COG reformer from the coke oven gas and/or the natural gas and partly from the top gas of the same DRI device. In this case, the reducing gas is mainly produced in approximately equal parts of coke oven gas and/or natural gas and from the top gas of the reduction device.

In particular, the reducing gas is produced from the top gas of the reduction device by condensation of water vapor and precipitation of carbon dioxide.

According to the invention, in particular, the coke oven gas and/or the natural gas is/are mixed with the gas containing steam and carbon dioxide and the resulting gas mixture as well as oxygen is introduced in the COG reformer. Mainly, the resulting gas mixture and the oxygen are introduced into the lower region of the COG reformer, while a subset of the oxygen is introduced into the central region of the COG reformer.

For the regulation, the outlet gas of the COG reformer is mainly supplied with a subset of coke oven gas and/or natural gas.

It has been proved as advantageous, that the gas mixture of the coke oven gas and/or the natural gas, the water vapor and the carbon dioxide-containing gas is heated before the introduction to the COG reformer, wherein its temperature is adjusted depending on the methane content.

For the introduction of the gas mixture from the coke oven gas and/or the natural gas, the water vapor, the carbon dioxide gas and the oxygen, the use of at least one double lance is preferable, wherein on the inner lance tube and in the space between the inner tube and the outer tube, the oxygen and the gas mixture in the COG reformer are introduced.

Therefore, the top gas from a DRI reduction device, which is preferably cooled and scrubbed in a gas scrubber, is used as the gas containing steam and carbon dioxide. By adjusting the temperature of the flow water and for the gas scrubbing, the steam/carbon dioxide ratio can be shifted depending on the methane content of the preheated mixed gas that is introduced to the COG reformer and on the process conditions in favor of one or the other component. The ratio of water vapor/carbon dioxide in the mixed gas also affects the ratio of hydrogen/carbon monoxide in the reducing gas. With a very large proportion of natural gas and thus also with a larger proportion of methane in the gas mixture, this water vapor and carbon dioxide-containing gas mixture can also be added to the steam.

As it is stated above, it is preferable that one part of the reducing gas that is supplied to the DRI reduction device is produced in the COG reformer, while another part is produced from the top gas of a DRI reduction device by the condensation of water vapor and the precipitation of carbon dioxide. The reducing gas that is produced from the top gas, is then heated in a heat exchanger at the required temperature, admixed with the reducing gas from the COG reformer and then introduced as a reducing gas of the same reduction device and then used for the production of porous iron.

For the removal of carbon dioxide from the top gas, conventional methods such as PSA (Pressure Swing Absorption) or chemical methods can be used.

In the DRI reduction device that is mentioned here, porous iron is produced, in which process at elevated temperatures, H₂ and CO react directly with iron oxides, without melting iron ore or pellets.

As already mentioned above, the mixture of coke oven gas and/or natural gas, water vapor and carbon dioxide-containing gas is heated before its introduction to the COG reformer. This serves in keeping the amount of oxygen for the COG reformer, the water vapor and carbon dioxide content of the reducing gas, low. The appropriate preheating temperature should, however, be chosen in a way that no soot problems occur in any of the corresponding facilities.

In the case of failure on the coke system, the coke oven gas can be replaced by the natural gas or added to the coke oven gas and be converted to reducing gas in the COG reformer. In both cases, the amount of water vapor and carbon dioxide-containing gas that is supplied to the coke oven gas or the natural or the corresponding gas mixture and the temperature of the preheated gas mixture can be adapted to the methane content of the gas mixture concerned, thereby preventing the thermal decomposition of methane and soot formation in the heat exchanger for preheating and in the COG reformer.

According to the invention, the objective of this method is preferably to replace the expensive natural gas with coke oven gas at the production of porous iron and to dispense with a reformer with its expensive catalyst tubes as well as the also costly catalyst of the state of the art. This inventive method allows for this waiver, even if the reducing gas is produced in the case of a coke oven gas lacking, from a mixture of coke oven gas and natural gas.

According to the invention, in this method the coke oven gas and/or the natural gas is heated by partial oxidation of the higher hydrocarbons and methane by oxygen to H₂ and CO and partly by combustion of the combustible gas components to water vapor and carbon dioxide to a temperature so high that with the gas mixture introduced higher hydrocarbons and methane with the retained in the gas mixture and formed in the COG reformer water vapor and carbon dioxide are reacted and decomposed to form H₂ and CO. In this case, higher hydrocarbons, methane and other organic constituents are decomposed to such an extent that the reducing gas thus prepared can be used directly or mixed with the reduction produced and heated from the top gas of the reduction device as reducing gas to produce porous iron in a reduction device.

Specifically, it is provided to heat the gas mixture to a relatively high temperature (in the heat exchanger) to keep the oxygen consumption for the COG reformer and the water vapor and carbon dioxide content in reducing gas, low.

The introduction of the gas mixture together with the oxygen, mainly takes place in the lower region of the COG reformer by at least one double lance. The gas mixture is therefore injected directly next to the oxygen, so that at high temperatures, higher hydrocarbons and methane are converted directly by reactions with the oxygen, water vapor and carbon dioxide to H₂ and CO.

The inventive method is thus relatively simple and very flexible in terms of quality and composition of the feed gas. It represents an alternative to the production of reducing gas without the use for a nickel catalyst.

According to the invention this method lends itself to replacing conventional, more complex and expensive apparatus not only of the construction of the new systems, but also at high natural gas prices in operational systems. Most facilities of the DRI systems in operation can still be used with little effort for conversion and minor adjustments to new process conditions. For this purpose, for example, the known catalytic reformer can be reduced, and the catalyst can be emptied. The catalytic tubes can be used to heat the reducing gas, in particular from the carbon dioxide precipitation system. Also, a heat exchanger, in which the feed gas for the catalytic reformer is preheated, can be used for preheating the gas mixture which is supplied to the COG reformer.

However, if too much hydrogen sulfide content is present in the coke oven gas, it is necessary to desulfurize the coke oven gas before it is introduced to the COG reformer to bring the sulfur content in the porous iron to an acceptable, for the still mill, level.

The present invention further relates to a system for producing sponge iron, in particular for carrying out the method described above, with a DRI reduction device and an introduction device of reducing gas into the DRI reduction device.

According to the invention, the system is characterized by what it further comprises a COG reformer in which the coke oven gas and/or natural gas, the water vapor and carbon dioxide-containing gas, which is top gas from a DRI reduction device, are reformed to the reducing gas.

The system mainly has a device for discharging and processing the top gas of the DRI reduction device to the reducing gas and for supplying the same reduction device together with the reducing gas from the COG reformer. In this way, the proportion of the reducing gas from the COG reformer and the proportion of the top gas of the DRI reduction device with respect to the reducing gas to be generated can vary.

The device for removing and supplying the top gas, mainly has a device for the condensation of water vapor and for the precipitation of carbon dioxide. Furthermore, the system mainly has a device for mixing the coke oven gas and/or the natural gas with the water vapor and carbon dioxide containing gas (top gas from the DRI reduction device) prior to the introduction into the COG reformer.

Specifically, it is provided that the system comprises means for introducing the obtained gas mixture and oxygen into the lower region of the COG reformer and means for introducing oxygen into the middle part of the COG reformer. Preferably, it has at least one double lance for introducing the gas mixture and oxygen into the COG reformer. The double lance that is designed as a double tube has, in particular, a water cooled inner tube enveloped by a protective tube.

In the development of the invention, the system has means for supplying a subset of coke oven gas and/or natural gas to the outlet gas of the COG reformer. Furthermore, it has means for heating water vapor and carbon dioxide containing gas (top gas from a DRI reduction device) prior to the introduction into the COG reformer.

The system that is designed according to the invention, has a DRI reduction device which preferably consists partly of a preheated mixture of coke oven gas and/or natural gas, water vapor and carbon dioxide containing gas in a COG reformer with the introduction of oxygen reducing gas and partly from the top gas supplied to the same DRI reduction device and then heated reducing gas and is used to produce porous iron.

Specifically, the system also includes means for preheating the gas mixture for the COG reformer. In addition to the mentioned devices for the condensation of water vapor from the top gas and the precipitation of carbon dioxide from the same, preferably provided are the means for heating the produced reducing gas to the required temperature. In addition to the devices used for injecting the gas mixture and the oxygen into the COG reformer, the devices for adding the coke oven gas and/or the natural gas into the outlet line of the COG reformer for adjusting the methane content of the reducing gas for the reduction device are preferably provided.

The invention will be explained below with a reference to an implementation in conjunction with the detailed drawings. The single FIGURE shows a schematic representation of the structure of a system for the production of porous iron with a DRI reduction device and a COG reformer.

In the lower region of the COG reformer 1 is a mixture of coke oven gas and/or natural gas—introduced by a line 7—water vapor and carbon dioxide containing gas from a top gas scrubber 6—introduced via a line 8—preheated in a heat exchanger 2 and injected via a line 9 and double lances 10 in the lower region of COG reformer 1 and converted by blowing the bulk of oxygen via double lances 10—introduced via a line 11—to the reducing gas. The blowing of the oxygen through the inner tube and the supply of the preheated mixed gas over the space between the inner and outer tube of the double lance 10. A smaller amount of oxygen—supplied through the common oxygen line 11—is injected through a simple, radially on the circumference of the COG reformer 1 mounted oxygen lances 12 into the central region of the COG reformer 1. The oxygen that is injected by oxygen lances 12 into the middle region of the COG reformer 1, serves in regulating the dome temperature of the COG reformer 1 and at the same time to mix the gas rising from the lower region of the COG reformer. The reducing gas produced in the COG reformer 1—is led out via a line 14—is added via a line 13 coke oven gas and/or natural gas to adjust the methane content of the reducing gas for the reduction device 5. The top gas led out of the reduction device 5 is washed and cooled in a gas scrubber 6, the water vapor formed in the reduction device 5 is condensing out. Most of the washed top gas is supplied via line 15 of a carbon dioxide precipitation system 3. The hydrogen and carbon monoxide-containing product gas is introduced via a line 16 to a reduction heater 4, heated to about 900° C. and is introduced via a line 17 to the reducing gas produced in the COG reformer 1, admixed via line 13, the reduction device 5 and introduced for producing porous iron, which is then discharged at the lower end or the reduction device 5. The carbon dioxide containing exhaust gas (tail gas), which is passed out via a line 18 from the carbon dioxide precipitation system 3, a smaller amount of the top gas from the gas scrubber 6 is added via a line 19 and fed to the reducing gas heater 4 as the heating medium.

The DRI system with a COG reformer 1 is a simple reformer in which the reducing gas is produced by reforming of the coke oven gas and/or the natural gas and by supplying an oxidizing agent and then it is supplied to the reduction device 5 via the line 13. The water vapor formed in the reduction device 5 is discharged in a gas scrubber 6 and the carbon dioxide that is formed, in a carbon dioxide precipitation system 3. The thus prepared reducing gas is heated in a heat exchanger 4 to the required temperature, mixed with the hot reducing gas from the COG reformer 1, supplied to the reduction device 5 and used for the production of porous iron. The amount of the water vapor and the carbon dioxide-containing gas supplied to the coke oven gas and/or the natural gas and the temperature to which the mixed gas is preheated before being blown into the COG reformer 1, are adjusted depending on the methane content of the mixed gas. A higher preheating temperature of the mixed gas leads to a lower oxygen consumption and a higher reduction potential of the produced reducing gas. A too high preheating temperature should, however, be avoided in order for the carbon precipitation to be kept low and for the sooting by thermal decomposition of methane in the heat exchanger 2 and in the COG reformer 1 low as well.

Since the steam is a much stronger medium against sooting than carbon dioxide, gas scrubbing 6 is operated at a higher methane content of the mixed gas with a higher feed water temperature, whereby more water vapor via line 8 is introduced with the coke oven gas and/or natural gas—introduced via line 7—into the preheater 2 is supplied, whereby the gas mixture in the preheater 2 can be heated to a higher temperature. At a very high methane content of the reducing gas also steam can be supplied to this subset of the top gas from the gas scrubber 6.

In order to mix the oxidizing agent and the preheated mixed gas as well as possible and to bring oxygen and methane to the reaction, both mediums are injected via radially circumferentially mounted double lances 10 in the lower region of the COG reformer 1, the oxygen over the inner tube and the preheated gas mixture are introduced through the space between the inner and outer tube of the double lances 10.

A smaller portion of the oxygen is blown through a plurality of oxygen lances 12 arranged on the circumference of the COG reformer at a higher speed into the middle region of the COG reformer 1 in order to better mix the gas components of the rising gas from the lower region of the COG reformer 1 and to increase the efficiency of it. The amount of oxygen supplied to the central region of the COG reformer 1 simultaneously serves to regulate the dome temperature of the COG reformer 1.

In order to ensure the fine adjustment of the methane content of the reducing gas for the reduction device 5, the outlet gas from the COG reformer 1 is supplied with a corresponding amount of coke oven gas or natural gas. The basic adjustment of the methane content is carried out by adjusting the ratio of the preheated mixed gas/oxygen, which are supplied via double lances 10 of the lower region of the COG reformer 1.

The adjustment of the temperature of the reducing gas mixture from the COG reformer 1 and from the carbon dioxide precipitation system 3 or from the gas heater 4, which is supplied to the reduction device 5, is done via the temperature of the reducing gas produced in the COG reformer 1.

In the operating DRI systems, in which the natural gas prices have become too high and the operation of the systems has become uneconomical, by converting the systems of catalytic reformers to COG reformers 1, a part of the catalytic reformer and the entire recuperation system can be put into further use. The use of the entire catalytic reformer is no longer necessary, since the heat requirement for heating the reducing gas from the carbon dioxide precipitation system to the required temperature, is not as high as for the decomposition of methane by water vapor and carbon dioxide in a catalytic reformer. It makes sense to use only a part of the catalytic reformer, including the catalytic tubes for heating the reducing gas from the carbon dioxide precipitation system to about 900° C. and the feed gas recuperating system from preheating the gas mixture for the COG reformer 1. However, small adjustments of the amount of the fuel gas, the ratio of air/fuel gas and the height of the temperatures of the combustion gas are required.

The inner tube of the double lance 10 designed as a double tube is furnished with a protection tube and can be made of highly heat-resistant steel or be designed as a water-cooled lance.

When converting a conventional DRI system based on natural gas, of coke oven gas and/or other gases, the catalyst reformer is replaced by a COG reformer 1, the catalytic reformer and recuperation of an operational system are rebuilt so that only one part of the catalytic reformer is maintained. The catalyst is thereby emptied, and the catalytic tubes are used for heating the reducing gas from the carbon dioxide precipitation system 3 and the feed gas preheater for preheating the mixed gas for the COG reformer 1. 

1. Method for the production of porous iron via a direct reduction process in which the reducing gas is introduced into the DRI reduction device and in which the direct reduction process is carried out, identified by the coke oven gas and/or natural gas by the addition of water vapor and carbon dioxide gas, which is top gas from a DRI reduction device (5), and from the oxygen in a COG reformer (1) is reformed to the reducing gas.
 2. Method according to claim 1, characterized in that the reducing gas is partially produced in the COG reformer (1) from the coke oven gas and/or natural gas and partly from the top gas of the same DRI reduction device (5).
 3. Method according to claim 2, characterized in that the reducing gas is produced in approximately equal parts from the outlet gas of the COG reformer (1) and from the top gas of the reduction device (5).
 4. Method according to one of the preceding claims, characterized in that the coke oven gas and/or natural gas is mixed with water vapor and carbon dioxide containing gas and the resulting gas mixture and oxygen are introduced into the COG reformer (1).
 5. Method according to claim 4, characterized in that the obtained gas mixture and oxygen in the lower region of the COG reformer (1) and a subset of the oxygen in the central region of the COG reformer (1) are introduced.
 6. Method according to one of the preceding claims, characterized in that the outlet gas of the COG reformer (1) is supplied to a subset of coke oven gas and/or natural gas.
 7. Method according to one of claims 4 to 6, characterized in that the gas mixture of coke oven gas and/or natural gas, the water vapor and carbon dioxide containing gas is heated prior to introduction into the COG reformer (1), wherein its temperature becomes adjusted depending on the methane content.
 8. Method according to one of the preceding claims, characterized in that at least one double lance (10) is used to introduce the gas mixture from the coke oven gas and/or natural gas as well as the water vapor, the carbon dioxide gas and oxygen, wherein the oxygen via the inner lance tube and the space between the inner tube and outer tube, the gas mixture in the COG reformer (1) are introduced.
 9. System for the production of porous iron, in particular for carrying out the process according to one of claims 1 to 8, with a DRI reduction device, characterized in that it further comprises a COG reformer (1) in which the coke oven gas and/or the natural gas, water vapor and carbon dioxide-containing gas, which is top gas from a DRI reduction device (5) and oxygen is reformed to the reducing gas, is provided.
 10. System according to claim 9, characterized in that in comprises a device for discharging and processing the top gas of the DRI reduction device (5) to the reducing gas and for supplying the same reduction device together with the reducing gas from the COG reformer.(1).
 11. System according to claims 9 to 10, characterized in that it comprises means for mixing the coke oven gas and the natural gas with the water vapor and carbon dioxide containing gas prior to introduction into the COG reformer (1).
 12. System according to claim 11, characterized in that it comprises the means for introducing the obtained gas mixture and oxygen into the lower region of the COG reformer (1) and the means for introducing oxygen into the middle part of the COG reformer (1).
 13. System according to one of claims 9 to 12, characterized in that it comprises the means for supplying a subset of coke oven gas and/or natural gas to the outlet gas of the COG reformer (1).
 14. System according to one of claims 9 to 13, characterized in that it has a device for heating the water vapor and the carbon containing gas, prior to introduction into the COG reformer (1).
 15. System according to one of claims 12 to 14, characterized in that it comprises at least one double lance (10) for introducing the gas mixture and oxygen in the COG reformer (1).
 16. System according to claim 15, characterized in that the double lance (10) designed as a double tube has a water-cooled inner tube enveloped by a protective tube. 